In the operation of a PABX, a subscriber dials a sequence of digits which are used by the PABX to complete a call to a line or trunk identified by at least some of the dialed digits. In the event the subscriber dials a prefix such as a 9, the PABX connects the calling subscriber line to an outgoing trunk, whereupon subsequently dialed digits following the 9 are passed to a central office to which the outgoing trunk is connected, for completion of the call.
The call numbering plan used in e.g. North America has used a convention which avoids confusion between area code numbers and central office numbers. Central office numbers have been restricted to the middle digit of the first three digits being between 2 and 9 inclusive, and the middle digit of area codes have been restricted to 0 or 1.
These restrictions have been removed. Area codes and office codes are now interchangeable.
In addition, in the past each telephone company had a monopoly in local service, and the dialed numbers automatically were detected by central office equipment owned by that telephone company. Long distance service was either provided by the local company (in some jurisdictions), or the long distance carrier was selected by the local telephone company. This monopoly has been broken, and the local subscriber can select the long distance company to use.
This creates a difficulty for PABXs, since they must be able to distinguish between digits dialed within a PABX, digits dialed which are to a called subscriber accessable via a local central office, and digits dialed which are to be connected via an alternate service provider, without being able to distinguish between area codes and other numbers based on the value of the second digit of the called number that is dialed, as in the past. Yet it is important to do so automatically, in order to take advantage of savings which can be obtained by use of alternate service providers.
For example, in order to route to central office 416 in the local area, a PABX owner would have to program an automatic route selection subsystem in the PABX to interpret the number 9416+4 digits to follow (the number 9 designating an out-of-PABX call) as meaning that the call should be routed to the local central office, and to interpret the number 9416+7 digits to follow as meaning that the call should be routed to an alternate service provider. It is generally not possible to do this on common PABXs, since the prefix 9 automatically causes the call to be routed to the local central office.